James G. Owen

Patterns of Mammalian Species Richness in Relation to Temperature, Productivity, and Variance in Elevation

In Texas, the relationship between environmental factors and species richness of all mammals, bats, rodents, and carnivores was investigated by multiple-regression methods. Variance in elevation was a strong, positive predictor of species richness for all mammals, bats (Chiroptera), and rodents (Rodentia). It was a significant but weak predictor for carnivores (Carnivora). Productivity had significant, negative regression coefficients for all mammals, bats, and rodents. Productivity was positive, significant, and relatively stronger for carnivores. Species richness of all mammals, rodents, and carnivores may follow a hump-shaped curve along a gradient of increasing environmental productivity. Mean-annual temperature range was significant with a negative sign for carnivores and with a positive sign for rodents. Mean intermonthly variability of precipitation had a positive and significant relationship to carnivores, but significant, negative coefficients for bats and rodents. Variation in all mammals and carnivore species richness, and in all mammals and bat richness, was significant, with positive signs, for maximum and minimum temperatures, of record, respectively. All mammals, bat, and rodent richness varied significantly, with negative signs, with the coefficient of variation of mean January temperature. Overall, the climatic-variability indices exhibited only weak control over density of mammalian species. With the exception of productivity their influence on current ecological hypotheses of species richness was complex. Statistical support, contradiction, and statistical lack of significance depended upon the particular combination of environmental factors and taxa under consideration. Results suggest that current ecological hypotheses of species richness often function in directions not concordant with predictions and are not satisfactory as general explanations of biotic richness for these vertebrate groups.

mtDNA perspective of chromosomal diversification and hybridization in Peters’ tent-making bat (Uroderma bilobatum: Phyllostomidae)

We compared sequence variation in the complete mitochondrial cytochrome‐b gene with chromosomal and geographical variation for specimens of Peters’ tent‐making bat (Uroderma bilobatum). Three different chromosomal races have been described in this species: a 2n = 42 race from South America east of the Andes, a 2n = 44 from NW Central America and 2n = 38 from the rest of Central America and NW South America. The deepest nodes in the tree were found within the South American race (42 race), which is consistent with a longer history of this race. Average distance among races ranged from 2.5 to 2.9%, with the highest amount of intraracial variation found within the 2n = 42 race (1.7%), intermediate values within the 2n = 38 race (0.9%) and lowest within the 2n = 44 race (0.5%). Variation among chromosomal races accounted for over 55% of molecular variance, whereas variation among populations within races accounted for 6%. The 2n = 38 and 2n = 44 races hybridize in the coastal lowlands of Honduras, near the Gulf of Fonseca. Introgression between these two races is low (two introgressed individuals in 45 examined). Clinal variation across the hybrid zone for the cytochrome‐b of U. bilobatum, is similar to clinal variation reported for chromosomes and isozymes of this species. Mismatch distribution analyses suggests that geographical isolation and karyological changes have interplayed in a synergistic fashion. Fixation of the alternative chromosomal rearrangements in geographical isolation and secondary contact is the most likely mechanism accounting for the hybrid zone between the 2n = 38 and 2n = 44 races. If a molecular clock is assumed, with rates ranging from 2.3 to 5.0% per million years, then isolation between these races occurred within the last million years, implying a relatively recent origin of the extant diversity in Uroderma bilobatum. None the less, the three chromosomal races probably represent three different biological species.

Reduced Number of Ribosomal Sites in Bats: Evidence for a Mechanism to Contain Genome Size

To better understand the organization of the genome of bats, we examined by in situ hybridization, the number of ribosomal DNA sites in 50 species of bats representing both suborders, 7 families, and 38 genera. Number of sites ranged from one to four pairs (average, 1.76) in bats, whereas the number of sites in 40 species of rodents ranged from two to ten pairs (average, 4.19). The possible relationship of a reduced number of sites to a smaller amount of DNA in the genome of bats is explored. We find little evidence to support the hypothesis that bats are retaining a fixed primitive condition of a low number of sites and we conclude that the most probable explanation is that bats, like other groups of mammals, have mechanisms that tend to increase the number of sites. However, the balance between mechanisms that increase and those that reduce the number of sites is more strongly in favor of reduction of sites than is characteristic of other mammals such as rodents.

Population and Geographic Variation of Peromyscus leucopus in Relation to Climatic Factors

Variation of nine cranial characters in the white-footed mouse, Peromyscus leucopus , from 38 localities in the state of Texas, was studied. Significant age effects were detected for each character, but no sexual dimorphism and no sex-by-age interaction. Mean locality scores on the first morphological principal component, a size vector, revealed a statewide trend for larger mice to occur in the Panhandle and Trans-Pecos and for smaller specimens to occur in southern and southeastern Texas. A projection of the locality centroids onto the first two morphological principal components did not exhibit a pattern that indicated subspecific differentiation. This interpretation was reinforced by the large overlap among 95% confidence ellipses for locality populations about their means and by chromosomal studies in the literature. Mantels test indicated that populations morphologically more different also occupy geographic areas climatically more different. The first two climatic principal components, derived from four climatic variables that measure mean annual temperature, mean July and January temperature, and mean annual precipitation, were interpreted as vectors representing temperature and precipitation, respectively. A multiple-regression analysis of size against the first two climatic principal components indicated that trends in size of white-footed mice follow Bergmanns Rule. There were significant inverse correlations between the first climatic principal component and seasonality of temperature, and between the second climatic component and seasonality of precipitation. This indicates that the traditional interpretation of Bergmanns Rule as a thermoregulatory response to cooler climates is confounded with adaptations to increased seasonality. Size allometry in white-footed mice may be a morphological response to harsh periods of low resource availability in seasonal climates. Seed productivity, a primary source of food for P. leucopus in Texas, may be correlated positively with size, and serve as an adaptive basis for selection of larger mice, with higher nutritional needs, in areas of higher seed resource availability.

Systematics of the Neotoma mexicana species group (Mammalia: Rodentia: Cricetidae) in Mesoamerica: new molecular evidence on the status and relationships of N. ferruginea Tomes, 1862

Abstract Analyses of the mitochondrial cytochrome-b gene were used to determine levels of genetic differentiation and patterns of relationship among members of the Neotoma mexicana (Mexican woodrat) species group in Mesoamerica. Three well-supported clades were obtained that conform to the species N. ferruginea Tomes, 1862, N. mexicana Baird, 1855, and N. picta Goldman, 1904. Neotoma ferruginea is the senior name for the clade that contains samples from southern Mexico and Nuclear Central America previously identified as N. isthmica Goldman, 1904, or as subspecies of N. mexicana (chamula Goldman, 1909; vulcani Sanborn, 1935). The phylogeographic pattern observed within the N. mexicana species group resembles that reported for other vertebrates co-distributed in mountains to the west (Trans-Mexican Neovolcanic Belt, Oaxacan sierras) and east (highlands of Chiapas, Mexico, and Nuclear Central America) of the Isthmus of Tehuantepec. Resumen Análisis filogenéticos del gen mitocondrial Citocromo b se utilizaron para determinar los niveles de diferenciación genética y los patrones de relaciones entre miembros del grupo de especies de Neotoma mexicana en Mesoamerica. Se obtuvieron tres clados bien soportandos que están conformados por las especies Neotoma ferruginea Tomes, 1862; N. mexicana Baird, 1855; y N. picta Goldman, 1904. Neotoma ferruginea es el nombre más antiguo para el clado que contiene muestras del sur de México y Centro América Nuclear que previamente fueron identificadas como N. isthmica Goldman, 1904, y una subespecie de N. mexicana (chamula Goldman, 1909; vulcani Sanborn, 1935). El patrón filogeográfico observado entre el grupo de especies de N. mexicana se asemeja al reportado para otros vertebrados co-distribuidos en las montañas del Oeste (Eje neovolcánico transversal, Sierras de Oaxaca) y el Este (tierras altas de Chiapas, México y Centro América Nuclear) del Itsmo de Tehuantepec.

Combining climatic variables: a case study from Texas, USA

Fifteen environmental variables of potential biological importance are studied. These variables quantify productivity, altitude, topographic heterogeneity, and a variety of climatic dimensions. Most of these variables are not normally distributed, according to the Kolmogorov‐Smirnov One‐Sample Test. Spearmans coefficient of rank correlation indicates that many of them are highly and significantly correlated. Principal components analysis reveals a gradient of increasing temperature from north to south and an orthogonal gradient of increasing precipitation across the state. Based on these vectors four climatic regions (warm‐moist, warm‐dry, cool‐moist, cool‐dry) are recognized in Texas. An interpretation of the biological meaning of the variables is presented. These data are of potential used as a resource base for studies of relationships between environmental factors and biological variation.